Synthesis, spectroscopy, magnetic and redox behaviors of copper(II) complexes with tert-butylated salen type ligands bearing bis(4-aminophenyl) ethane and bis(4-aminophenyl)amide backbones

Article abstract of DOI:10.1016/j.saa.2012.11.061

New salen type ligands, N,N′-bis(X-3-tert-butylsalicylidene)-4, 4′-ethylenedianiline [(X = H (1), 5-tert-butyl (2)] and N,N′-bis(X-3-tert-butylsalicylidene)-4,4′-amidedianiline [X = H (3), 5-tert (4)] and their copper(II) complexes 5-8, have been synthesi

Full text of DOI:10.1016/j.saa.2012.11.061

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopy, magnetic and redox behaviors of copper(II) complexes
with tert-butylated salen type ligands bearing bis(4-aminophenyl)ethane
and bis(4-aminophenyl)amide backbones
b
a
a
Veli T. Kasumov a, , Yusuf Yerli , Aysegul Kutluay , Mehmet Aslanoglu
⇑
a Department of Chemistry, Harran University, Osmanbey, 63300 Sßanlıurfa, Turkey
b Department of Physics, Gebze Institute of Technology, Gebze-Kocaeli, Turkey
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
New N,N-bis(X,3-t-
butylsalicylidene) separated by
bis(4-aniline)ethane/amide ligands
Cu(II) complexes.
"
"
"
Spectroscopic characteristics of all
compounds.
Effect of linkers on the magnetic
moments of Cu(II) complexes.
Chemical oxidation of synthesized
compounds.
a r t i c l e i n f o
a b s t r a c t
New salen type ligands, N,N0-bis(X-3-tert-butylsalicylidene)-4,40-ethylenedianiline [(X = H (1), 5-tert-
butyl (2)] and N,N0-bis(X-3-tert-butylsalicylidene)-4,40-amidedianiline [X = H (3), 5-tert (4)] and their
copper(II) complexes 5–8, have been synthesized. Their spectroscopic (IR, 1H NMR, UV/vis, ESR) proper-
ties, as well as magnetic and redox-reactivity behavior are reported. IR spectra of 7 and 8 indicate the
coordination of amide oxygen atoms of 3 and 4 ligands to Cu(II). The solid state ESR spectra of 5–8 exhib-
its less informative exchange narrowed isotropic or anisotropic signals with weak unresolved low field
Article history:
Received 11 August 2012
Received in revised form 7 November 2012
Accepted 16 November 2012
Available online 5 December 2012
Keywords:
ESR
Optical spectroscopy
Magnetic properties
Chemical oxidation
Electrochemistry
patterns. The magnetic moments of 5 (2.92 l l
B per CuII) and 6 (2.79 B per CuII) are unusual for copper(II)
complexes and considerably higher than those for complexes 7 and 8. Cryogenic measurements (300–
10 K) show weak antiferromagnetic exchange interactions between the copper(II) centers in complexes
6 and 8. The results of electrochemical and chemical redox-reactivity studies are discussed.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
steric and electronic effects of substituents of salicylic and aniline
rings are the key factors in various reactions such as symmetrical
The coordination chemistry of transition metal complexes with
salen-type ligands has achieved a considerable attention in the last
decades, because of their oxygen carrying reactivity, redox-chem-
istry, unusual magnetic and structural properties, as well as their
usage as models for metalloproteinase [1–5], as catalysts for the
oxidation and polymerization reactions [1,4–7]. Particularly, the
or asymmetrical salen catalysts, the epoxidation, polymerization
catalysts and the electron transfer reactivity of the metal com-
plexes with ligands bearing bulky tert-butyl for instance, in sym-
metrical or asymmetrical salen catalysts, in the epoxidation and
polymerization catalysts and in the electron transfer reactivity of
the metal complexes with ligands bearing bulky tert-butylphenol
moieties [4–6]. The oxidative chemistry of transition metal com-
plexes with proradical ligands, and the correlation of reactivity
with electronic structure have been an area of considerable
⇑
Corresponding author. Tel.: +90 414 318 3588; fax: +90 414 3440051.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.061

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V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
research interest [1,2]. One of the unique properties of complexes
bearing tert-butylphenol is that they easily generate relatively sta-
ble M(II)/M(III)-phenoxyl radical complexes upon chemical or elec-
trochemical oxidation [5–7]. Especially, the discovery of protein
radicals such as copper-containing enzymes (galactose and glyoxal
oxidases) and others [8] has stimulated the development of simple
model transition metal complexes bearing phenolic pendant arms,
which are capable of to generating M(II)/M(III)-phenoxyl radical
complexes upon oxidation [5–7]. In addition, recent studies have
revealed that the complexes of group 4 metals [6d,e,9] as well as
p-block metals [10–12] with bulky tert-butylated bi-, tri- and tet-
radentate phenoxyimine type ligands are one of the highly active
catalysts for olefin polymerization or a variety of asymmetric oxi-
dation, epoxidation and other reactions. Yoshida and co-workers
and others [10,11] have designed several new salen-type ligands
and their transition metal complexes in which two N,O-bidentate
chelating moieties are separated via aromatic backbones having a
central ACH2A, AOA, ASA, ASO2A, AC6H4AOAC6H4A and other
spacer groups. X-ray structural studies performed on M(II) com-
plexes of above salen type ligands revealed that the formed
chelates, in general, possess a binuclear or tetranuclear double-
helical structure which is mainly controlled by the intramolecular
300 K) magnetic susceptibility measurements were carried out
with a Quantum Design PPMS SQUID magnetometer operating at
a magnetic field of 10 kOe. The measured magnetic susceptibilities
were corrected for temperature-independent paramagnetisim and
ligand diamagnetism using Pascal’s constants. EPR spectra were re-
corded with a Bruker EMX X-band spectrometer (9.8 GHz) with
about 20 mW microwave power and 100 kHz magnetic field mod-
ulation. An EcoChemie Autolab-12 potentiostat with the electro-
chemical software package GPES 4.9 (Utrecht, The Netherlands)
was used for voltammetric measurements.
A platinum disk
(2 mm o.d.) was employed as a working electrode, a platinum coil
as a counter electrode, and an Ag/AgCl as reference electrode,
respectively. All measurements were performed in CH2Cl2 contain-
ing 0.05 M n-Bu4NClO4 as a supporting electrolyte at room temper-
ature (r.t.) and under nitrogen atmosphere.
Materials
All other chemicals were commercially available N,N-dimethyl-
formamide (DMF), dimethylulfoxide (DMSO), acetonitrile (MeCN),
chloroform, ethanol, methanol, 2,4-di-tert-butylphenol, copper(II)
acetate monohydrate, (NH4)2Ce(NO3)6, acetic acid, 4,40-diamino-
diphenylethane, 4,40-diaminodiphenylamide, 3-tert-butyl-salicyl-
aldehyde and n-Bu4NClO4 obtained from Aldrich Chemical Co,
Inc., and were used without further purification. The 3,5-di-tert-
butylsalicylaldehyde was synthesized according to a published
procedure [14].
p–p
stacking and CH–p interactions between the spacer groups
[10–12].
In the course of our interest in the structure, spectroscopic
behavior and redox reactivity of transition metal complexes with
redox-active ligands bearing tert-butylated phenol fragments
[13], we have interested in the coordination chemistry of tert-
butylated salen type ligands, in which salicylaldene moieties are
separated by backbones bearing bis(phenyl)ethane and bis(pheny-
l)amide groups. In this paper, we describe the synthesis, spectro-
scopic characterization, magnetic and redox reactivity behaviors
of the new salen type ligands 1–4 and their copper(II) complexes
abbreviated as 5, 6, 7 and 8 (Scheme 1).
Synthesis of ligands
General method for the synthesis of ligands. To a hot solution of
X-tert-butylsalicylaldehyde (8 mmol) in 50 ml absolute ethanol
was added the corresponding amine (4 mmol) in 30 ml ethanol
and then a few drops of formic acid as a catalyst. The yellow solu-
tion was further heated at reflux with stirring for about 8–10 h and
that a yellow precipitate was appeared. The volume of the reaction
mixture was reduced to ca. 30 ml and allowed to cool to r.t. The
resulting light yellow precipitate was filtered, washed with ethanol
and air-dried. The crude product was recrystallized from dichloro-
methane/methanol (1:1) solvent mixture.
Experimental
Methods
The elemental analyses (C, H, N) were performed at the Scien-
tific and Technological Research Center, Inönü University of Tur-
key. UV–vis spectra were measured on a Perkin–Elmer Lambda
25 spectrometer operating between 200 and 1100 nm. IR spectra
were recorded as KBr pellets on a Perkin–Elmer FTIR spectrometer
in the 450–4000 cmꢀ1 region. 1H NMR spectra were measured on a
Bruker Spectro spin Avance DPX-300 Ultra Shield Model NMR with
Me4Si as an internal standard in CDCl3 solutions. The room temper-
ature (r.t.) magnetic susceptibility was measured by using a
Sherwood Scientific magnetic balance. Variable temperature (10–
Bis(N-3-tert-butylsalicylidene-4-aminophenyl)ethane (1)
Yield 88%. M.p. 219 °C. Anal. calcd. for C36H42N2O2 (532.76): C,
81.17; H, 7.57; N, 5.26%. Found: C, 81.05; H, 7.34; N, 5.17%. IR
[KBr pellet,
m
(cmꢀ1)]: 2865, 2913, 2957 (CAH, tBu), 2650 (OH,
intramolecularly OHꢁ ꢁ ꢁN), 1618 (C@N), 1595, 1506 (C@C). 1H
NMR (300 MHz, CDCl3): d = 13.73 (br shoulder, 2 H; OH), 8.68 (s,
2 H; CH@N), 7.36 (d, 2 H; J = 1.6 Hz, meta-coupled doublet in sali-
cylic ring), 7.22 (t, J = 4. 45 Hz, 2 H in salicylic ring) 7.088 (d, 2 H;
J = 1.2 Hz, meta-coupled doublet in salicylic ring), 7.26–7.43 (m.
8 H; Ph), 2.96 (t, J = 13.6 Hz, 4 H, ACH2ACH2A), 1.4372 (s, 18H,
C(CH3)3).
R
R
R
Ligands :
O
O
X= CH2CH2
HO
N
Cu
R = H (1); R = C(CH3)3 (2)
N
N
X = CONH
Bis(N-3,5-di-tert-butylsalicylidene)-4-aminophenyl)ethane (2)
Yield: 94%. M.p. 244. Anal. calcd. for C44H56N2O2 (644.93): C,
81.95; H, 8.75; N, 4.34%. Found: C, 81.05; H, 8.54, 4.45%. IR [KBr
R = H (3); R = C(CH3)3 (4)
Cu(OAc)2
EtOH/CHCl3
X
X
X
Complexes :
t
pellet,
m
(cmꢀ1)]: 2867, 2909, 2951 (CAH, Bu), 2560–2780 (intra-
X = CH2CH2
N
molecularly H-bonding OH), 1619 (C@N), 1585, 1506 (C@C). 1H
NMR (300 MHz, CDCl3): d = 13.81 (br shoulder, 2 H; OH), 8.68 (s,
2 H; CH@N), 7.36 (d, 2 H; J = 1.0 Hz, meta-coupled doublet in sali-
cylic ring), 7.088 (d, 2 H; J = 1.2 Hz, meta-coupled doublet in sali-
cylic ring), 7.36–7,53 (m. 8 H; Ph), 2.95 (t, J = 13.6 Hz, 4 H,
ACH2ACH2A), 1.437 (s, 18H, C(CH3)3), 1.298 (s, 18H, C(CH3)3).
R = H (5), R = C(CH3)3 (6)
N
N
Cu
OH
X = CONH
O
O
R =H (7), R = C(CH3)3 (8)
R
R
R
Scheme 1. Chemical structures of ligands (1–4) and their complexes (5–8).

V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
205
Bis(N-3-tert-butylsalicylidene)-4-aminophenyl)amide (3)
ethanol under reflux. These ligands (Scheme 1) contain bis(X-
tert-butylsalicylidene) chelating sites in which the imine groups
are linked via bis(4-aminophenyl)ethane (1 and 2) and bis(4-
aminophenyl)amide (3 and 4) spacers. As predicted by quantum
chemical semi-empirical AM1 calculations the above ligands pre-
dominantly adopt anti-opened conformer structure (Figs. 1 and
2). Complexes 5, 7 and 8 are satisfactory soluble in the strong sol-
vents such as DMF, CH2Cl2 and CHCl3 but the solubility of complex
6 is very poor in the above solvents probably due to its polymeric
nature. The coordination ability of 3 and 4 ligands having CONH
amide linkage group is quiet interesting with respect that they
are potentially pentadentate, and could be to form an O2N3 or
O3N2 type donor trigonal–bipyramidal or square-pyramidal Cu(II)
complexes. The structures of all the synthesized compounds (1–8)
are confirmed by UV/vis, IR, 1H NMR spectral studies and elemental
analysis. Despite numerous attempts, single crystals of 5, 7 and 8
complexes suitable for X-ray crystallography could not be
obtained.
Yield: 82%. M.p. 224–225 °C. Anal. calcd. for
C35H37N3O3
(547.68): C, 78.27; H, 8.09; N, 6.36%. Found: C, 77.42; H, 7.97; N,
6.84%. 1H NMR (300 MHz, CDCl3): d = 13.58 (br shoulder, 2 H;
OHꢁꢁꢁN), and 13.51 (br, 1 H; AHNAC@O), 8.70 (s, 1 H; CH@N),
8.71 (s, 1 H; CH@N), 7.9 (d, 2 H; J = 8.4 Hz, meta-coupled doublet
in salicylic ring), 7.8 (d, 2 H; J = 15.9 Hz, meta-coupled doublet in
salicylic ring), 7.25 (m, (eight doublets), 8 H; phenyl rings), 1.436
(s, 18H, C(CH3)3), IR [KBr pellet,
m
(cmꢀ1)]: 3378 (NAH), 2872,
2909, 2952 (CAH, tBu), 2740 (OH, OHꢁꢁꢁN), 1659(C@O), 1616
(C@N), 1594, 1509 (C@C).
Bus(N-3,5-di-tert-butylsalicylidene)-4-aminophenyl)amide (4)
Yield: 87%. M.p. 270 °C. Anal. calcd. for C43H53N3O3 (659.89): C,
78.27; H, 8.09; N, 6.36%. Found: C, 77.42; H, 7.97; N, 6.84%. 1H NMR
(300 MHz, CDCl3): d = 13.48 (br shoulder, 2 H; OHꢁꢁꢁN), and 13.64
(br, 1 H; AHNAC@O), 8.70 (s, 1 H; CH@N), 8.71 (s, 1 H; CH@N),
7.9 (d, 2 H; J = 8.4 Hz, meta-coupled doublet in salicylic ring), 7.8
(d, 2 H; J = 15.9 Hz, meta-coupled doublet in salicylic ring), 7.25
(m, (eight doublets), 8 H; phenyl rings), 1.437 (s, 18H, C(CH3)3),
IR and 1H NMR spectra 1–4 ligands
1.298 (s, 18H, C(CH3)3), IR [KBr pellet,
m
(cmꢀ1)]: 3452 (NAH),
2868, 2907, 2954 (CAH, tBu), 2560–2780 (OH, OHꢁꢁꢁN),
1682(C@O), 1619 (C@N), 1585, 1506 (C@C).
Selected characteristic IR absorption frequencies of ligands (1–
4) are given in the Experimental section. The
mC@N and mOAH
stretching’s of the 1–4 free ligands appeared within 1616–1619
and 2600–2800 cmꢀ1 ranges as strong and medium intensity
bands, respectively. Narrow bands at 3378 and 3452 cmꢀ1 ob-
served in the IR spectra of the amide linkage 3 and 4 ligands,
Synthesis of complexes
General method for the synthesis of copper(II) complexes (5–8)
respectively, are assigned to amide
mNAH stretching vibrations. A
A
solution of copper(II) acetate monohydrate (0.055 g,
broad strong bands at 1659 and 1682 cmꢀ1 detected in the spectra
0.275 mmol) in methanol (5 ml) was added, under constant stir-
ring, to a hot suspension of ligand (0.275 mmol) in EtOH/CHCl3
(50/20 ml). After heating (70–80 °C) the solution for a 30–40 min,
the solution volume was reduced to ca. 20 ml and then allowed
to cool to r.t. A green precipitate has been formed immediately
for the complexes 5, 6 and 7 upon the addition of Cu(II) acetate
to the corresponding ligand solution. The resulting green precipi-
tate was collected by vacuum filtration and washed with methanol
and ethanol and dried at 45–50 °C. 5: yield: 0.3 g, 91%. M.p. (de-
comp) > 260 °C. Anal. calcd. for C36H38N2O2Cu (594.24): C, 72.76;
of 3 and 4 can be attributed to mC@O frequencies, respectively. The
1H NMR spectral results obtained for 1–4 ligands in CDCl3 with
their assignments is given in ‘Synthesis of ligands’. 1H NMR signals
were assigned on the basis of their integral intensity, spin–spin
coupling patterns and chemical shift values.
AM1 optimization for 2 and 4
In order to identify the most stable conformers for 2 and 4 li-
gands, the Semiempirical calculations on ligands 2 and 4 were car-
ried out using the AM1 method. The geometry optimization
revealed that the most stable minimum energy geometry for both
H, 6.45; N, 4.71%. Found: C, 72.81; H, 6.32; N, 4.63%. IR [KBr pellet,
t
m
(cmꢀ1)]: 2868, 2907, 2954 (CAH, Bu), 1613 (C@N), 1588, 1506
(C@C), 1536 (CAO), 562, 533, 507 (CuAO and CuAN). Magnetic
moment per CuII
(l
eff): 2.92 B 295 K. 6: yield: 0.3 g, 91%. Mr. (de-
l
camp.) > 300 °C. Anal. calcd. for C44H54N2O2Cu (706.46): C, 74.81;
H, 7.71; N, 3.96%. Found: C, 72.91; H, 7.50; N, 4.26%. IR [KBr pellet,
t
m
(cmꢀ1)]: 2867, 2905, 2953 (CAH, Bu), 1614 (C@N), 1588, 1506
(C@C), 1529 (CAO), 631, 538, 512 (CuAO and CuAN). Magnetic
moment per CuII
(l
eff): 2.79 lB 7: yield: 73%. M.p. (de-
.
comp) > 300 °C. Anal. calcd. for C35H35N3O3Cu (609.22): C, 69.00;
H, 5.79; N, 6.89%. Found: C, 69.07; H, 5.87; N, 6.97%. IR [KBr pellet,
t
m
(cmꢀ1)]: 3314 (NAH), 2867, 2907, 2952 (CAH, Bu), 1647 (C@O),
1610 (C@N), 1589, 1507 (C@C), 1533 (CAO), 553, 530 (CuAO and
CuAN). Magnetic moment per CuII
eff): 1.85 B at 292 K. 8: Yield:
73%. M.p. (decomp) > 300 °C. Anal. calcd. for 43H51N3O3Cu
(721.43): C, 71.59; H, 7.13; N, 5.82%. Found: C, 69.87; H, 6.87; N,
(l
l
C
5.97%. IR [KBr pellet,
m
(cmꢀ1)]: 3316 (NAH), 2867, 2904, 2953
(CAH, tBu), 1669 (C@O), 1615 (C@N), 1585, 1507 (C@C), 1528
(CAO), 553, 530 (CuAO and CuAN). Magnetic moment per CuII
(leff): 1.79 lB at 298 K.
Results and discussion
The
bis(X-tert-butylsalicylidene)-4,40-(diaminophenyl)ethyl-
ene/amide ligands 1–4 were synthesized in good yields by simple
Schiff base condensation of one equivalent of the respective dia-
mines and two equivalents of tert-butylated salicylaldehydes in
Fig. 1. Two main possible conformers for 1 (X = ACH2CH2A) and 2 (X = ACONHA)
ligands.

206
V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
observed for theoretically optimized geometry of similar salen
type ligands [10,12d].
IR spectra of 5–8 complexes
The IR spectra of the complexes 5–8 show that the
m
(C@N)
stretching is blue shifted to 1600–1615 cmꢀ1
, while mOAH
(2600–2800 cmꢀ1 due to intramolecularly H bonded OHꢁꢁꢁN in 1–
4) disappears in all complexes, indicating the coordination of the
imine nitrogen and deprotonated phenolic oxygen atoms to cop-
per. The coordination mode of 5–8 was further supported by the
appearance of new strong band at 1525–1536 cmꢀ1 attributable
to
450–600 cmꢀ1 assignable to
[16]. The comparison of the free ligands amide linkage
bands of 3 (3378 cmꢀ1) and 4 (3452 cmꢀ1) with those for their 7
m
CAO [15,16] and medium intensity absorptions in the region
CuAO and CuAN stretching modes
NAH
m
m
m
and
8 complexes show low frequency shift to 3314 and
3316 cmꢀ1, respectively. Similarly, strong broad bands of the free
ligands 3 (1659 cmꢀ1) and 4 (1682 cmꢀ1) are shifted to 1646 and
1664 cmꢀ1 in the spectra of complex 7 and 8, accordingly. The ob-
Fig. 2. Electronic spectra of 1 and 2 ligands in EtOH.
served blue shifts in mC@O and mNAH, indicate the coordination of
the amide linkage to Cu(II) ion occur via carbonyl oxygen atom
[17]. These observations indicate that amide linkage ligands act
as pentadentate N2O3 chelators. Thus, comparison of the free li-
gands IR spectra with those for their complexes suggests pentaco-
ordinate geometry (trigonal bipyramidal or square pyramidal)
with CuN2O3 coordination chromophore for the 7 and 8 complexes.
compounds is non-planar anti-opened conformer (Fig. 3). The low-
est total energy (Etot) and the heats formation energy ( Hf) for both
conformers are calculated by using AM1 method. The AM1 opti-
mized energies for conformer 2 (Etot = ꢀ10800 kcal/mol, Hf =
ꢀ18.2884 kcal/mol) and conformer 4 (Etot = ꢀ10660.41 kcal/mol,
D
D
D
Hf = ꢀ33.55 kcal/mol) have been predicted. The dihedral angles
between neighboring benzene rings, which are labeled as a–d
(Fig. 3), in both conformers also have been evaluated by AM1 cal-
Electronic spectra of 1–4 ligands
culations. The dihedral angles between benzene rings, ua/b, ub/c
and uc/d are 51.028°, 28.102° and 50.173°, respectively, for con-
former of 2. Similarly, the dihedral angles between the four ben-
Electronic absorption spectral data of 1–4 in ethanol and DMF
are given in Table 1 and Fig. 3. The strong absorption bands ob-
served below 300 nm are assigned to intraligand
p ?
pꢂ transi-
, )
tions in the aromatic rings (1A1g ? 1E1u 1A1g ? 1B1u 1A1g ? 1B2u
,
zene rings were as following:
ua/b = 44.432°, ub/c = 62.66° and uc/
d = 45.009° for conformer 4. While the total energy for 2 and 4 sta-
ble conformer is close to each other, the heats of formation energy
for 4 is significantly higher than that for 2. Thus, in both stable con-
formers the aromatic rings are not coplanar. The AM1 optimized
conformations for 2 and 4 are in good agreement with those
and the C@N chromophores [3b,18,19a]. The absorption band ob-
served in ligands spectra within the range of 327–340 nm is most
probably due to the transition of n ? pꢂ of imine group [18]. The
intense bands appeared in the 357–370 nm region, according to
their higher extinction coefficients (log
e )
= 4.34–4.88 Mꢀ1 cmꢀ1
Fig. 3. The AM1-optimized conformers of 2 and 4 ligands. The C, O, N and H are represented by green, red, blue and white color spheres, respectively. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version of this article.)

V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
207
Table 1
triplet-state species [14b,21] were observed. Powdered ESR spec-
trum of 5 shows poorly resolved axial spectrum with g// of ca.
2.243 and nearly isotropic singlet at giso = 2.074 with an exchange
Electronic spectral data of 1–8 and their oxidized species under ambient conditions.
Compound Solvent
Electronic spectra, k (nm), (log
209, 225, 239a, 275, 311, 324, 350
277(4.6), 313(4.7), 326(4.73), 350(4.73), 430a
e )
, Mꢀ1 cmꢀ1
narrowed line width (
spectrum of 6 along with intense isotropic singlet at giso = 2.108
Hpp = 115 G) also exhibits very weak and poorly distinguishable
DHpp) of 68.5 G [Fig. 6A (5)]. The powder
1
EtOH
DMF
DMF + Ox. 269, 325, 347, 500a
(D
2
EtOH
207(4.6), 228(4.4), 278(4.4), 313(4.5), 327(4.5),
357(4.4), 430
broad pattern approximately around g-factor of 2.5, as presented in
Fig. 6A (6). It is interesting that when the ESR spectrum of this sam-
ple was scanned at 120 K, the intensity of the above broad feature
is increased and a broad signal becomes clearly apparent at
g = 2.566 [Fig. 7A (6a)]. In addition, an isotropic singlet at
giso = 2.108 [Fig. 6A (6)] was converted to anisotropic broad pattern
DMF
280(4.42), 314(4.42), 328(4.48), 357(4.47)
DMF + Ox. 278, 314, 326, 355, 530a
3
EtOH
214(4.6), 236(4.4), 281(4.6), 338(4.2), 363(4.2),
460a(2.2)
DMF
282(4.7), 322a, 335a, 366(4.8), 460a
DMF + Ox. 280, 363, 530a
with g// around of 2.222, g\ = 2.069 and line width
DHpp of 120 G
4
5
EtOH
DMF
207, 220a, 238a, 288a, 340, 369, 480a
286(4.6), 345(4.79), 369(4.83), 480a(2.11)
[Fig. 7A (6a)] at 120 K, suggesting a change in geometry around
the copper atom. Fig. 7A (6b) shows the twofold increased left site
part of the spectrum presented in Fig. 7A (6a). The significant in-
crease in the intensity of signal at g = 2.566 upon cooling to
ca.120 K suggests the existence of ferromagnetic interactions be-
tween adjacent Cu(II) centers in paramagnetic species which
formed the above EPR signal. The r.t. ESR spectrum of powder sam-
DMF + Ox. 268, 310, 340a, 365a, 530a
CH2Cl2
DMF
296(4.57), 412(4.26), 670(268)
300(4.5), 406(4.3), 667(2.32)
DMF + Ox. 277, 311, 324, 350, 550b
6
7
CH2Cl2
DMF
CH2Cl2
DMF
245, 299, 363a, 423
300, 327a, 357, 420a, 460a
240a(4.44) 298(4.39), 408(4.03), 672(2.28)
316(4.4), 414(4.1), 520b, 686(2.45)
ple of
7 is characterized by an axial g tensors with g//
DMF + Ox. 282, 338a, 365, 390a, 430a, 470a, 560a
(2.248) > g\(2.074) > ge [Fig. 6B (7)]. The observed trend,
8
CH2Cl2
DMF
245a(4.53), 316(4.62), 431(4.32), 665a(2.48)
316(4.6), 426(4.3), 530b, 700a(2.4)
1
g// > g\ > ge, suggests a ðdx2ꢀy2
Þ
ground state in a copper centers
DMF + Ox. 266, 311, 362a, 490 a, 636, 750b, 840b
for above complexes [21]. The exchange-coupling parameter,
G = (g// ꢀ ge)/(g\ ꢀ ge), reflects the exchange interaction between
the copper(II) centers in polycrystalline solids [22]. According to
Hathaway and Billing [22], if G > 4 the exchange interaction is neg-
ligible. The obtained value G = 3.62 for powder samples of 7, sug-
gests the presence of an exchange coupling interaction between
Cu(II) centers in the solid state [22]. On the other hand, in the pow-
der spectrum of 8 along with a very weak intensity two g// compo-
nents with A// about of 150 G, the anisotropic splitting of g
components with g// = 2.151 and g\ = 2.094 (gav = 2.113) can be
Ox. = (NH4)2Ce(NO3)6.
a
Shoulder.
Very weak shoulder.
b
can be assigned to the charge transfer transitions within the delo-
calized pꢂ system in the molecular structure [18,19]. The weak
shoulder appeared at around 430 nm which is assignable to
n ? pꢂ transition in their keto-amine tautomeric forms, indicates
that only minor amounts of keto tautomer are present in these li-
gands [19].
determined from Fig. 6B (8). The observed trend, g// > g\ > ge, sug-
1
gests a ðdx2
Þ
ground state in a copper centers for above com-
ꢀy2
plexes [21c]. It is necessary to note that, since IR spectra of 7 and
8 indicate the N2O3 donor site for these complexes, it is likely that
the coordination mode around copper(II) center can adopt a square
pyramidal or trigonal–bipyramidal geometry in above complexes.
Electronic spectra of 5–8
Because of poor solubility of 6 in CH2Cl2, CHCl3, DMSO and DMF
solvents, we could obtain its electronic spectra only within 245–
460 nm region (Table 1). All 5, 7 and 8 complexes in CH2Cl2, CHCl3
and DMF solutions exhibit very similar intense bands in the 300–
433 nm region (Table 1 and Figs. 4 and 5). The intense broad band
However, the ESR spectra of
7 and 8 having signals with
g// > g\ > 2.03 and UV–vis spectral data are typical for square-pyra-
midal geometry [21].
Unfortunately, because of low solubility of the 5–8 complexes,
the solution ESR spectra in dichloromethane could be obtained
only for 5 and 8 compounds at room temperature. As can be seen
from Fig. 8, the expected a four-line copper hyperfine spectrum,
which is usually detected in the solution ESR spectra of Cu(II),
has not been appeared in the spectra of 5 and 8 complexes. The
solution ESR spectra of these complexes in CH2Cl2. display a broad
component at g = 2.546 and narrow ones at giso = 2.096
within 406–431 nm can be assigned to Cu(II) (d )-to-phenolate
p
(pz) MLCT transition [20]. The electronic spectra of all complexes
in the above solvents exhibit two similar doublets within 300–
432 nm region assignable to overlapped n ? pꢂ and LMCT transi-
tions [3,19,20]. In visible region, 5, 7 and 8 exhibit a broad absorp-
tion about 660–700 nm (Table 1). The band appeared at 667 nm
(e
= 211 Mꢀ1 cmꢀ1) in the spectrum of 5 is assigned to d–d transi-
tions in the tetrahedrally distorted square-planar geometry. The
band observed at 686 and 700 nm in the visible spectrum of 7
and 8, respectively, can be assigned to the d–d transitions in the
square-based pyramidal geometry since CuN2O3 chromophore is
proposed for the complex 7 and 8 according to their IR and UV/
vis spectra [20–22]. A square-based pyramidal geometry with an
approximate C4v symmetry is proposed for these complexes since
the absorptions in the 650–700 nm region in the visible spectra
of 7 and 8 complexes lie within range typical for tetrahedrally dis-
torted square-planar and square-based pyramidal geometries [21]
and on the basis of their solid state ESR spectra (g// > g\) results.
(
D
Hpp = 109.8 G) for 5 in Fig. 7B (5) and the same parameters with
g = 2.551 and giso = 2.113 ( Hpp = 117.4 G) for 8 in Fig. 7B (8),
D
which are not typical for mononuclear Cu(II) compounds. We sup-
pose that the observed solution’s spectral feature, most probably, is
originated from polynuclear Cu(II) polyhedrons and without X-ray
structural data, the accurate interpretation of the above spectral
features is not easy.
Magnetic characterization of 5–8 complexes
All magnetic measurements were corrected for diamagnetism
using Pascal’s constants. The r.t. effective magnetic moments, leff
,
ESR characterization of 5–8 complexes
for 5 (2.92 B) and 6 (2.79 B) per Cu(II) ions are considerably high-
l
l
er even the highest possible limit value (2.3 MB). Nevertheless, the
absence of ferromagnetic interaction in these polycrystalline sam-
ples was supported by the field dependence measurements of the
The polycrystalline ESR spectra of all 5–8 were obtained at
300 K (Fig. 6). No half-field
DMs = 2 lines typical for binuclear

208
V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
Fig. 4. Electronic spectra of 6 and 8 complexes: (a) spectra of 6 and 8 in CHCl3. Inset: (b) spectrum of 8 in DMF.
Fig. 5. Electronic spectrum of 5 (I, II and III) and its one-electron oxidized species [5 + 2 equiv Ce(IV)] in DMF and at r.t (IV–XII): (a) spectra IV–VIII scanned subsequently via
50 s; (b) spectra IX–XII monitored after 15, 25, 50 and 70 min, respectively; XIII-spectrum of diluted sample XII. The arrow indicates the direction of increasing absorbance.
magnetization M at 300 K. According to literature [10,11], bi-, tri-
or tetranuclear helical structure can be predicted for complexes
5, 7 and 8 and a polymer type structure might be proposed for
complex 6 because of its practically non-solubility feature. On
0.0005 emu K molꢀ1 = 0.000463 emu K molꢀ1 and h =
, N
a
ꢀ0.2998 0.009 for 6 and C = 0.1500 emu K molꢀ1, N = 0.000785
a
emu K molꢀ1 and h = ꢀ1.389 K for 8 were determined. Negative
Weiss temperatures suggest the presence of weak antiferromag-
netic interactions between Cu(II) ions within the 6 and 8 com-
plexes. The cryomagnetic behavior of 6 is shown in Fig. 8a, in
the other hand, the r.t.
agrees well with the values reported for square-planar or five-
coordinate Cu(II) compounds, i.e. 1.7–2.0 B [23]. Magnetic suscep-
leff for 7 (1.85 lB) and 8 (1.79 lB) per Cu(II)
l
terms of the molar magnetic susceptibility (
versus T. The
mT value of 2.124 cm3 molꢀ1 K (
300 K of 6 decreased monotonously with lowering temperature
vm) and
vmT product
tibility data for polycrystalline samples of complexes 6 and 8 were
collected over temperature range 10–300 K at a magnetic field of
10 kOe. The obtained data for the samples of 6 and 8 complexes
v
l
eff = 4.124
lB) at
to 0.766 cm3 molꢀ1 K (
leff = 2.477 lB) at 10 K, due to temperature-
can be fitted by the relation
constant, h is Weiss constant and N is temperature-independent
v
m(T) = C/(T ꢀ h) + N , where C is Curie
independent paramagnetisim (N ) associated with the CuII ion
a
a
[24]. The cryomagnetic property of complex 8 is shown in Fig. 8b
a
paramagnetisim. The fitting procedure leads to C = 0.7192
in the form of vm and vmT vs. T plot. The vmT product of 8 was

V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
209
Fig. 6. Powder ESR spectra of 5 and 6 (A) and spectra of 7 and 8 (B) at 300 K.
Fig. 7. (A) Powder ESR spectrum of 6 (6a) at 120 K. Inset: (6b) the twofold enhanced spectrum of the low field site part of spectrum 6a. (B) Solution ESR spectra of 5 and 8 in
CH2Cl2 at 300 K.
determined to be 0.4155 cm3 molꢀ1 K (
Upon cooling mT product also decreases monotonously with
decreasing temperature from 0.4155 emu K molꢀ1
eff = 1.79 lB
at 300 K to 0.1578 emu K molꢀ1
eff = 1.124 B) at 10 K. It follows
that two hypotheses can be proposed to explain the decrease in
mT when the temperature is lowered. The first one attributes this
l
eff = 1.82
l
B) at 300 K.
lar to our ligands, form a binuclear and a tetranuclear Cu(II)
complexes and exhibits temperature-independent eff in the range
5–295 K and, in addition, possesses normal magnetic moments ex-
v
l
(
l
)
(l
l
pected for uncoupled Cu(II) centers [9c].
v
Chemical oxidation of 1–8 compounds
phenomenon to the thermally induced spin conversion in the com-
plex, present as an impurity. The second one postulates the exis-
tence of a significant temperature-independent paramagnetism,
which would be due to a relative closeness of thermally unpopular
ted excited levels with respect to the ground state [25]. The second
postulate is valid for our complexes. For comparison magnetic
behavior, we also prepared non tert-butylated analogs of 5 and 6
and found that its effective magnetic moment per Cu(II) was 2.22
In previous studies of the electron transfer behavior of N-Alkyl
(Aril)-3,5-di-tert-butylsalicylaldimine, 3,5-di-tert-butyl-salen type
ligands and their transition metal complexes, it has been found
that the generation of relatively stable uncoordinated phenoxyl
or coordinated M(II)-phenoxyl radical complexes were detected
upon one-electron oxidation of the above compounds [12b–d].
The oxidative behavior of 1–8 compounds was investigated by
in situ UV/vis spectral measurements. The results of UV/vis spec-
tral changes were observed during the chemical oxidation of 1–8
are given in Table 1 and Figs. 5, 9 and 10. Upon one-electron oxida-
tion of ligand 1–4 ligands (ꢃ0.005 M) with 1 equiv of Ce(IV) in
lB 290 K, which also suggests the existence of ferromagnetic inter-
actions between Cu(II) centers. It should be noted that the ligands
having AC6H4A(CH2)nA, n = 1, 2 asymmetric spacer groups be-
tween 3,5-di-tert-butylsalicylidene moieties, which are very simi-

210
V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
Fig. 8. Temperature dependence of the molar magnetic susceptibility
of mT product.
vm for 6 (a) and 8(b). Solid line represents a fit by Curie–Weiss law. Inset: the temperature dependence
v
Fig. 9. (a) Electronic spectra of 1 and its oxidation products ((1 + 1 equiv Ce(IV)); (b) electronic spectra of 2 and its one-electron oxidation products (2 + 1 eqv ox.); spectra
presented as a–e in figure (b) are recorded with period 45 s after addition of Ce(IV) to a solution of ligand 2. All spectra are scanned in DMF.
Fig. 10. UV/vis spectra of complex 8 (1–3) in DMF. The changes in the electronic absorption spectrum of 8 (2.3 ꢄ 10ꢀ3 M) during the chemical oxidation for 8 + 2 equiv Ce(IV)
system in DMF solution and aerobic conditions. Each spectrum from 4 to 7 is scanned with 50 s period, 7, 8 and 9 monitored via 5, 10 and 15 min, respectively. The spectra 10,
11 and 12 obtained with 20 min period. Spectra 13 and 14 scanned by dilution of 12.

V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
211
Fig. 11. (Left) A cyclic voltammogram of complex 5 in dichloromethane and 0.05 M n-Bu4NClO4 as supporting electrolyte. Scan rate 100 mV/s. (Right) Cyclic voltammo-grams
of complex 5 in dichloromethane at increasing scan rates. Scan rates: 100, 200, 300 and 400 mV/s.
DMF, a color of ligand solution simultaneously turns from yellow
to red along with the disappearance of the original ligand band
at ca 430 and the blue shifts in the bands with k < 366 nm, the
appearance of a new shoulder in the range 500–530 nm were ob-
served (Table 1 and Fig. 9). Upon successive scanning of 2 + 1 equiv
Ce(IV) system, the increase of the intensity of a shoulder appeared
around 530 nm (Fig. 9b) indicates that the generated oxidized spe-
cies are relatively stable under above conditions. The appearance of
new bands in the spectra of the oxidized ligands is assigned to phe-
noxyl radical species.
Upon chemical oxidation of 5 and 7 with 2 equiv of Ce(IV) in
DMF, along with simultaneously disappearance of the d–d band
at 667 and 686 nm, the appearance of a new weak shoulder at
around 550 and new maxima bands in UV region were detected
(Table 1, Fig. 5). Under above conditions chemical oxidation of
complex 8 with 2 equiv of Ce(IV) in DMF, along with immediate
disappearance of the bands at 426 and 700 nm, the appearance
of new absorptions at 286, 324 and 368 nm were detected
(Fig. 11). Upon successive scanning of 8 + 2 equiv Ce(IV) system
along with the appearance of new weak shoulders at around 750
and 840 nm, as well as the appearance of another new shoulder
around 650 nm were observed. Upon further scanning the trans-
formation of a shoulder at 650 nm to maximum at 636 nm can
be clearly seen from Fig. 10. The disappearance of the d–d transi-
tions is indicative to reduction of Cu(II) to Cu(I) or one-electron
oxidation of Cu(II) to Cu(III) with concomitant generation of the
coordinated phenoxyl radical to copper(II) center. The reappear-
ance of new bands in the visible region around 750 and 840 nm,
and then disappearance with the following appearance of a new
band at 636 nm can be interpreted by the conversion of one inter-
mediate Cu(II) complex, probably, with trigonal bipyramidal sym-
metry to another Cu(II) complex with square-pyramidal geometry.
NClO4 as the supporting electrolyte. The results of cyclic voltam-
metry (CV) measurements are given in Table 2. Representative
voltammograms are shown in Fig. 11. The CV of copper complex
5–8 exhibit quasi-reversible waves at of versus Ag/AgCl which
can be ascribed to Cu(III)/Cu(II) couples. As shown in Fig. 11a, com-
plex 5 displays an oxidation peak at Epa = 0.948 V with a corre-
sponding reduction peak at Epa = 0.865 V. The peak separation
for this couple (DEp) is 83 mV. The most significant feature of this
redox couple is the one-electron transfer electrochemical process
of the formation of Cu(III)/Cu(II). The difference between forward
and backward peak potentials can provide a rough evaluation of
the degree of the reversibility of one electron transfer reaction.
The electrochemical behavior of 5 can be explained by having the
presence of noninteracting copper ion centers where the redox
process for each occurs at about the same potential. The appear-
ance of only one wave has been previously reported for a number
of multicopper systems [26–29]. Generally, one wave is seen if
DEp
between two or more separate processes such as CuIII–CuIII + -
eꢀ ? CuIII–CuII + eꢀ ? CuII–CuII is <ꢃ100–200 mV. The analysis of
cyclic voltammetric responses with the scan rate varying 100–
400 mV/s gives the evidence for a quasi-reversible one electron
oxidation (Fig. 11b). The ratio of cathodic to anodic peak height
was less than one. However, the peak current increases with the
increase of the square root of the scan rate. The Ip/v1/2 value is al-
most constant for all scan rates. This establishes the electrode pro-
cess as diffusion controlled [30]. The separation in peak potentials
increases at higher scan rates. These characteristic features are
consistent with the quasi-reversibility of Cu(III)/Cu(II) couple.
The other reduction peaks are listed in the Table 2 and are probably
due to the ligand moiety of complex. The electrode potentials of
the rest of the copper complexes are given in Table 2.
Conclusion
Electrochemistry
A new flexible Schiff base ligands [bis-(N,N0-X-tert-butylsalicy-
lidene)-4-amino-phenyl]ethylene (1 and 2) and bis-[(N,N0-3,5-di-
tert-butylsalicylidene)-4-aminophenyl] amide (3 and 4) ligands
and their copper(II) complexes (5–8) have been synthesized. Their
spectroscopic (IR, 1H NMR, UV/vis, ESR), magnetic behaviors and
redox reactivity were studied. On the basis of IR, UV/vis and solid
state ESR spectroscopic results for amide linkage 7 and 8 com-
plexes a square based pyramidal geometry is proposed. The r.t.
Electrochemical properties of copper complexes were studied
on a Pt disc electrode in dichloromethane containing 0.05 M n-Bu4-
Table 2
Voltammetric data for 5–8 copper(II) complexes.
Complex
Epa (V)
Epc (V)
D
Ep (mV)
Epc (V)
5
6
7
8
0.948
1.068
0.968
1.117
0.865
0.846
0.863
0.436
83
ꢀ0.794
ꢀ0.450
ꢀ0.870
ꢀ1.000
222
105
681
effective magnetic moments,
per Cu(II) ions are considerably higher even the highest possible
limit value (2.3 B) for Cu(II) compounds. Variable temperature
(10–300 K) study of 6 and 8 shows that their magnetic moments
leff, for 5 (2.92 lB) and 6 (2.79 lB)
l
Supporting electrolyte = 0.05 M n-Bu4ClO4, scan rate = 50 mV/s.

212
V.T. Kasumov et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 203–212
[10] (a) N. Yoshida, N. Ito, K. Ichikawa, J. Chem. Soc., Perkin Trans. 2 (1997) 2387–
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linearly decreased from 4.12 to 1.79 lB at 300 K to 2.48 and 1.13 lB
at 10 K, respectively. Chemical oxidation of ligands lead to the for-
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noxyl radicals. Also, except 8, along with the disapperance of the
d–d and LMCT bands, a weak shoulder around 500–560 nm was
observed for the oxidized complexes. The electrochemical behavior
of copper complexes also exhibit the presence of noninteracting
copper(II) centers where the redox processes occur. Thus, accord-
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oxidation can be proposed for these complexes.
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Acknowledgements
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Financial support by the Harran University Sciences Research
Council (HUBAK is gratefully acknowledged. The authors are grate-
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ligands.
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